Various types of droplet ejecting printer technologies have been or are being developed. One such technology, acoustic ink printing (AIP), uses focused acoustic energy to eject marking material (generically referred to herein as ink) onto a recording medium. More detailed descriptions of AIP can be found in U.S. Pat. Nos. 4,308,547, 4,697,195, and 5,028,937, and the citations therein.
While AIP appears promising, most acoustic ink printers rely on selectively applying RF drive voltages to piezoelectric transducers to control ejection. The switching of RF drive voltages complicates AIP.
Another droplet ejection control technique is described in co-pending U.S. patent application Ser. No. 07/940,596 entitled, "Droplet Ejection by Acoustic and Electrostatic Forces." In that application, droplet ejection is induced by the simultaneous application of RF voltage to a transducer (to generate sufficient acoustic energy to create a "mound" of an ink) and of voltage to an electrode near the mound (to create an electrostatic field). Since the RF voltage by itself is insufficient to eject a droplet, the application of the electrode voltage controls ejection.
While combining RF drive signals with electrostatic fields is promising, since a system as described in Ser. No. 07/940,596 depends on additive forces it may not be optimum. Additive forces are a problem since the size and trajectory of ejected droplets depend upon the interactions of difficult to control variables such as the RF voltage, the resulting acoustic energy, the focus of the acoustic energy, the effect of the electric field on the ink, and the viscosity of the ink. Since uncharged fluids are attracted to electric fields, the use of electric fields to stop ejection, rather than to trigger it, using a system such as that described in Ser. No. 07/940,596 is not simple.
However, in the 1940's Winslow reported that electric fields increase the viscosity of some fluid; this property is called electrorheology. Importantly, an increase in viscosity makes acoustic droplet ejection more difficult. More recently, Professor Frank Filisko of the University of Michigan has reported on electrorheological fluids comprised of aluminosilicate ceramic particles suspended in various oils. Further, various mixtures of mineral oil and corn starch are electrorheological (about 1 to 5 parts by weight of corn starch to mineral oil gives good results). Other electrorheological fluids include corn starch in silicon oil, and a composition made by "belt mixing chlorinated polypropylene or copolymers of ethylene methacrylic acid at 115.degree. C. with carbon black and isopar, a mineral oil, in an attiter containing stainless steel beads." The last two fluids are from a conference on electrorheology held Aug. 7-9, 1989 at the McKimmen Center, Raleigh, N.C. Finally, D. G. Frood of Lakehead University, Canada, has reported electrorheology in "various concentrations of potato starch in 50 centistoke silicone oil" (electroviscous effects are seen for fields of about 400 to 2000V/mm).
Therefore, it would be advantageous to utilize electrorheology in acoustic ink printing, particularly in a manner such that the switching of RF drive voltages is not required.